This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions with coated carrier components, or coated carrier particles that can be prepared by, for example, dry powder processes. More specifically, the present invention relates to compositions, especially carrier compositions comprised of a core, a polymer composite coating comprised of a matrix resin into which a conductive component will fully disperse, a conductive component, and a resin wherein the conductive component is fully excluded from, that is, microscopic examination of the composite indicates that all of the conductive component is present in the first resin and the conductive component is not present in the second, or exclusion phase, resin. Further, the triboelectric potential of the exclusion phase resin is high, imparting a triboelectric charge to a toner in excess of about 30 microcoulombs per gram, preferably greater than about 40 microcoulombs per gram, at concentrations of, for example, below 40 percent by weight, and preferably about 20 percent by weight, of the exclusion phase resin in the polymer composite. The confinement of the conductive component to one of the two resins in the composite reduces the concentration of the conductive component necessary to achieve high composite conductivity. Preferred ranges for each of the components in the composite are from about 5 to about 17 percent by weight of the conductive component, from about 5 to about 45 percent by weight of the exclusion phase polymer, and from about 90 to about 38 percent by weight of the matrix resin into which the conductive component is fully dispersed. More specifically, the polymer composite is comprised of matrix resin, such as a crosslinked polyester-polyurethane, available as Envirocron, a conductive filler, of for example carbon black, and an exclusion phase resin, such as crosslinked melamine resin particles.
The primary function of the matrix resin is to bond the composite constituents to the carrier core surface such that the composite is substantially uniform and mechanically tough; that is, resistant to separation from the core via chipping, flaking, or peeling and resistant to abrasion, or gradual wearing of the surface of the polymer on the carrier. The matrix resin should combine excellent mechanical properties with acceptable flow and coating characteristics. The primary function of the conductive component is to create a conductive path through the polymer coating to allow charge transport between contacting carrier particles, and the primary function of the high triboelectric exclusion phase, for example the melamine crosslinked resin particles is to provide a high triboelectric charging site on the surface of the carrier beads to charge toner, and at the same time segregate carbon black (or other conductive fillers) from the polymer matrix resin. Exclusion of the conductive additive from the exclusion phase resin results in a more conductive carrier at a given concentration of carbon black in the composite by concentrating the conductive phase in the resin matrix; specifically, 10 percent carbon black by weight in the composite results in a carrier conductivity of 6.times.10.sup.-12 mho/cm in the absence of an exclusion phase resin and 9.8.times.10.sup.-9 mho/cm with the inclusion of 40 percent (percent by weight) melamine exclusion phase resin. Alternatively, segregation of the conductive additive from the exclusion phase resin results in equivalent conductivity at a lower concentration of carbon black in the composite; specifically 15 percent carbon black by weight in the composite results in a carrier conductivity of 3.times.10.sup.-9 mho/cm in the absence of an exclusion phase resin and 10 percent carbon black by weight in the composite results in a conductivity of 9.8.times.10.sup.-9 mho/cm with the inclusion of 40 percent melamine exclusion phase resin. Although the invention is not be limited by theory, it is believed that the presence of the exclusion phase resin concentrates the conductive additive in the matrix resin phase increasing the contacts between adjacent conductive additive particles, effectively lowering the conductive additive concentration at which the composite will conduct electricity, defined as the percolation threshold. Specifically, the concentration of conductive additive drops from greater than about 15 percent by weight, and preferably greater than about 20 percent by weight in the absence of an exclusion phase resin, to less than about 12 percent, and preferably less than about 8 percent by weight in the presence of an exclusion phase resin, when the exclusion phase resin is present in the composite at a concentration of greater than about 10 percent, preferably greater than about 20 percent, and most preferably greater than about 30 percent by weight of the total composite.
Also, and more specifically, in the presence of the conductive component there is enabled carriers with increased developer triboelectric response, for example equal to or greater than about 25 microcoulombs per gram of toner, preferably about 35 to about 80 microcoulombs per gram of toner, and most preferably greater than 45 microcoulombs per gram of toner, at relative humidities of from about 20 to about 90 percent, improved image quality performance, excellent high conductivity ranges of from about 10.sup.-10 to about 10.sup.-7 (ohm-cm).sup.-1, and the like. An important advantage associated with the carriers of the present invention include a high triboelectrical charge, for example a carrier tribo range of from about a plus (positive charge) 20 to about 100, and preferably from about a positive 20 to about a positive 90 microcoulombs per gram, and most preferably from a positive about 25 to a positive about 50 microcoulombs per gram.
The carrier particles of the present invention can be selected for a number of different xerographic copiers and printers, such as high speed color xerographic copies, printers, digital copiers, and more specifically wherein colored copies with excellent and substantially no background deposits are achievable. Developer compositions comprised of the carrier particles illustrated herein are generally useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging and printing processes, and digital processes. Additionally, the invention developer compositions comprised of substantially conductive carrier particles are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition and conductive component selected.
Advantages of the carriers of the present invention include in embodiments high robust carrier tribo charge of a positive value, high toner tribo charge of preferably a negative value, excellent admix, for example, from about 1 to about 30 seconds as determined in the charge spectrograph, and the like. More specifically, the toner tribo is from about a minus 50 to about a minus 150, from about a minus 55 to about a minus 90, or from about a minus 60 to about a minus 85, with corresponding positive tribo charges for the carrier. The tribo charges can be determined by a number of known methods, such as the use of a Faraday Cage.
Other advantages of the present invention include increased resistance of the carrier to mechanical aging in a xerographic environment and a decreased sensitivity of the carrier triboelectric value to the relative humidity of the environment. With respect to high toner tribo charge of a negative value, this property is important to xerographic, especially color applications, primarily because there is enabled development of toner particles into regions of the imaging member, such as a photoreceptor where strong fringe electrical fields exist, that is, at the borders of solids areas and lines. Developing toner particles through these fringe fields minimizes or eliminates the untoned part of the image which appears between two adjacent colors in an image.